A 'month,' as in Genesis 29, signifies the end of an earlier state and the beginning of a following state, thus a new state. 'A month' signifies a full state.

(Odkazy: Arcana Coelestia 3814)

Firstborn, in a supreme sense, represents the Lord as to divine celestial love, and also peope who were of the celestial church. The sanctification of the firstborn, as in Exodus 13, signifies faith in the Lord. The firstborn, in the spiritual sense of the Word, is good, for with infants the good of innocence is first infused by the Lord, by virtue of which man first becomes a man. Since good is of love, and man does not reflect upon his own love, but only upon the thoughts of his memory, and since good has not at first a quality, but acquires one when it is formed in truths, and without a quality nothing is perceived, hence it was unknown that good was the primary principle, or firstborn. Good is first conceived from the Lord in man, and is produced by truths, in which good is manifested, in its own form and effigy. Firstborn, as in Psalm 89:28, signifies the Lord's humanity.